It is desirable to scan the contents of objects such as, for example, baggage containers, liquid containers etc, at security and customs checkpoints to gain information about content, for example to obtain an indication that the contents of the object do not constitute a threat to security or a breach of customs regulations. It is also desirable to scan the contents of objects for other purposes such as quality control, content verification, degradation monitoring etc.
It is known that information useful in this regard may be obtained from an analysis of radiation received at a detector after interaction with the object. To ensure that the contents of an object are what they are claimed to be, it may be useful to scan the object and contents so that a high energy ionising radiation beam traverses a cross section of the object. It can be possible to obtain an indication of the materials composition and/or identity from a numerical analysis of the resultant transmitted radiation beam intensity data and a comparison of the results of that analysis with a reference dataset relating equivalent pre-stored data to a range of expected constituent materials of a known composition and/or of a known identity to which unique data can be attributed. Information regarding the composition and/or identity of an object under test and/or its contents may then be inferred from a match of test data to pre-stored data.
The interaction of high energy ionising radiation with an object is the subject of a number of physical mechanism including various modes of absorption, diffraction, elastic and inelastic scattering which have been characterised to a greater or lesser degree. Many of these mechanisms vary characteristically with material content in a manner which is dependent upon the radiation wavelength/frequency.
Instrumentation has been developed which is intended to examine objects non-invasively to identify content materials, in particular for example liquids, within sealed containers by referencing such characteristic energy selective information to a database of such data for known materials. According to techniques disclosed for example in our co-pending International Patent Application Nos. WO2009/024817, WO2009/024818, and WO 2008/142446 a container may be irradiated with a beam of radiation and the transmission or other characteristics of the container and its liquid contents measured using an energy selective detector such as cadmium telluride or germanium.
The transmission or other characteristics of various materials could be recorded and held in a database. The database could then be used for comparison with in situ scanning of objects to look for matched transmission characteristics and, therefore, target materials could be identified.
A problem arises in that precise transmission data may be influenced by the apparatus conditions when it is collected. All instruments will vary to some degree however high a tolerance is designed in principle. Errors in build, such as component offsets and misalignments, cannot be eliminated. Sources necessarily exhibit some variation. Detector performances and sensitivities can never be perfectly matched. Even an individual instrument can vary in performance over time, eg as operating conditions change, if it is moved etc. Although good practices and standard processes can reduce these variations between instruments and apparatus conditions, they cannot be entirely eliminated.
Reference data from a reference database generated on another instrument or otherwise in other apparatus conditions can only be used on a current instrument to tolerances that take due account of these variations between apparatus conditions. This may be inadequate for the levels of precision required to differentiate multiple materials with closely related characteristics. The alternative, to populate the current instrument with a bespoke generated reference database, would be time consuming and, therefore, unsuitable for many intended uses, such as in airport security where time is often of the essence.
There is a need for an improved a method of calibration of a dataset produced by an apparatus for scanning an object for non-invasively identifying the composition of its contents so as to make the data more readily available to other apparatus or in other apparatus conditions. In a particular preferred case there is a need for an improved method of populating such an apparatus with dynamically calibrated reference information derived from a over an operationally practical timescale. In a further preferred case there is a need for an improved method of populating a central reference database with data generated from multiple apparatus according to a common reference standard.